The present invention relates to a top-loading monopole antenna element and, more particularly, to the configuration of a monopole antenna element being low in price and usable in a wide band, for which fine adjustment of input impedance and resonance frequency is easily carried out.
Computer networking has been advancing rapidly along with the widespread use of computers such as PCs. Based on this, attention is currently focused on wireless LAN systems that are capable of establishing computer networks without links between computers by a network cable or the like.
Such wireless LAN system includes an antenna element (hereinafter simply referred to as antenna) for sending/receiving data exchanged between computers via radio waves. The antenna is mainly comprised of a circular plate made of conductors such as metals, a printed board including a dielectric board, and a conductive monopole that is joined with the center of the circular plate at one end and with the printed board at the other end.
In the following, a description will be given of the configuration of a conventional antenna element. FIG. 1 is a perspective view showing the configuration of a conventional antenna element which, for example, sends/receives data using a 2.4 GHz or 5 GHz band.
The antenna shown in FIG. 1 comprises a circular plate 901 made of conductors such as metals, a monopole 902 that is also made of conductors such as metals, and a printed board 903. The top of the monopole 902 is joined with the center of the circular plate 901.
The printed board 903 includes a conductive ground board 904 as an upper layer and a dielectric board 905 as a lower layer. Further, there is arranged a microstrip line 906 made of a conductor under the dielectric board 905. The ground board 904 is provided with a pattern clearance hole 907. The lower end of the monopole 902 extends through the pattern clearance hole 907 without touching the ground board 904, and is joined with the microstrip line 906 by soldering or the like at a power supply point 908.
Next, a description will be given of the principle of electrical operation of the conventional antenna. FIGS. 2(a) and 2(b) are diagrams showing the current distribution of the conventional antenna. Referring to FIG. 2(a), a current i0 runs through the monopole 902. Meanwhile, there are currents i1 to i4 flowing in the circular plate 901. As can be seen in FIG. 2(a), by the use of the circular plate 901, the original current i0 is distributed over the circular plate 901. Accordingly, the antenna can be shortened or lowered.
The currents in the circular plate 901, which are represented by the four currents i1 to i4 as a pattern in FIG. 2(a), flow in all directions from the center of the circular plate 901. Considering field emissions from these currents, it is readily understood that the current i0 generates vertical polarized waves being parallel to the current i0, which are uniformly radiated in a horizontal plane. Meanwhile, electromagnetic radiations generated from the currents i1 to i4 vectorially counteract each other. Thus, it turns out that there is no field radiation having horizontal components.
On the other hand, considering the range of frequencies, the antenna almost invariably resonates at a wavelength resulting from the current running through the length of l0 and l1 since the radius of the circular plate 901, where the currents i1 to i4 flow, is invariant. Thus, the conventional antenna exhibits a bandwidth capability equal to or lower than that of monopole antennas having no plate.
FIG. 3 is a graph showing the return loss of the conventional antenna by actual measurement. The antenna has the configuration illustrated in FIG. 1. The diameter of the circular plate is approximately 6 mm. The distance between the circular plate and the upper surface of the printed board, that is, the length of the monopole except for the part inserted in the printed board is 5.5 mm. In addition, the microstrip line is arbitrarily provided with stubs thereon to carry out adjustment of return loss characteristics. As can be seen in FIG. 3, the conventional antenna is operational in a bandwidth of 280 MHz, which constitutes about 5.1% of the frequency spectrum.
In the communication facilities of various nations, frequency assignments for the wireless LAN system are tend to expand because of the convenience or usability of the system. Accordingly, there is need for techniques to produce wideband antennas used in the wireless LAN system.
Additionally, with the popularization of the wireless LAN system, it becomes an important issue for manufacturers to reduce the cost of hardware. Therefore, considerable importance is attached to ideas on the antenna structure for realizing inexpensive antennas.
Besides, adjustments of analog RF characteristics are carried out by necessity at the production inspection section of each manufacturer. Should the adjustments be accomplished in antennas, the direct first run rate (yield ratio) can be improved. As a result, the total cost of the antennas can be reduced.
However, in the configuration of the above-mentioned conventional antenna, a current flows the uniform length of the monopole and the radius of the circular plate. Therefore, the antenna does not resonate at a number of wavelengths.
In addition, while the combination of the monopole and circular plate is created by cutting off them from single piece of substance, by molding, or by soldering after manufacturing them separately, any of the methods entails relatively high manufacturing cost. Moreover, since it is difficult to carry out fine adjustment for antenna part after manufacturing, the direct first run rate (yield ratio) cannot be improved effectively, which precludes the supplies of low cost antennas.
For example, there is described a top-loading antenna in Yasuto Mushiake,xe2x80x9cAntennas and Radio Propagation,xe2x80x9d pp. 69-70 (Corona Inc.). This type of antenna has a load such as a circular ring at its top. Although the antenna was originally devised for the purpose of keeping an elevation angle of radiation small or reducing high-angle radiation, it also had the effect of shortening or lowering the antenna itself Since the antenna had been used in medium waves with low frequency, the antenna was chiefly aimed at improving the elevation angle of radiation and shortening or lowering its height. Consequently, the uniformity of directivity in a horizontal plane (rotational symmetry), cross-polarized wave characteristics, wideband applicability, and easy adjustment were not taken into consideration.
It is therefore an object of the present invention to provide an antenna element being usable in a wider band as compared to the conventional antennas.
It is another object of the present invention to provide an antenna element which is simple in structure and low in price.
It is yet another object of the present invention to provide an antenna element, for which fine adjustment of input impedance and resonance frequency is easily carried out.
In accordance with the first aspect of the present invention, to achieve the above objects, there is provided an antenna element comprising: a top plate; a printed board; and a monopole that is joined with the top plate at one end and with the printed board at the other end in such a manner as to hold the top plate in a fixed posture toward the printed board; wherein, the top plate is symmetrical with respect to two orthogonal axes which are perpendicular to the thickness of the top plate and passing through the center of the area of the top plate; and the lengths of the axes are different from each other.
In accordance with the second aspect of the present invention, in the first aspect, the top plate has an elliptical shape.
In accordance with the third aspect of the present invention, in the first aspect, the top plate has a shape selected from polygons including rectangle, rhombus, hexagon and dodecagon.
In accordance with the fourth aspect of the present invention, in the third aspect, the angles of the polygon-shaped top plate are all or partly rounded off
In accordance with the fifth aspect of the present invention, there is provided an antenna element comprising: a top plate; a printed board; and a monopole that is joined with the top plate at one end and with the printed board at the other end in such a manner as to hold the top plate in a fixed posture toward the printed board; wherein, the top plate is integrally formed with the monopole from a flat plate; the top plate is symmetrical with respect to two orthogonal axes which are perpendicular to the thickness of the top plate and pass through the center of the area of the top plate; the lengths of the axes are different from each other; the top plate and the monopole are formed by means of bending the monopole out of the flat plate; and the monopole-bending creates a cutaway in the flat plate.
In accordance with the sixth aspect of the present invention, in the fifth aspect, another cutaway is provided in the flat plate, and the top plate is substantially symmetrical with respect to the longitudinal axis of the cutaways and an axis running at right angles thereto, which are perpendicular to the thickness of the top plate.
In accordance with the seventh aspect of the present invention, in the fifth or sixth aspect, the cutaways are formed longitudinally or transversely of the top plate.
In accordance with the eighth aspect of the present invention, in the fifth aspect, the top plate has an elliptical shape.
In accordance with the ninth aspect of the present invention, in the fifth aspect, the top plate has a shape selected from polygons including rectangle, rhombus, hexagon and dodecagon.
In accordance with the tenth aspect of the present invention, in the ninth aspect, the angles of the polygon-shaped top plate are all or partly rounded off.
In accordance with the eleventh aspect of the present invention, in one of the first to fifth aspects, the top plate can be bent from a bend axis including a point where one end of the monopole is joined.
In accordance with the twelfth aspect of the present invention, in the eleven aspect, the top plate can be bent into a V-shape or an inverted V-shape.
In accordance with the thirteenth aspect of the present invention, in one of the first, fifth, sixth, seventh and eleventh aspects, the monopole is provided with a bend part which can be bent out of the plane of the monopole to form a recess.